A printed circuit may include a nonconductive substrate having defined metallic conductive circuit paths formed on one or both major surfaces of the substrate. Holes may be formed through the substrate to provide for electrical connection with later-assembled leads of components in the case of a single-sided printed circuit. Holes formed through the substrate of a double-sided printed circuit eventually become (1) metallic-lined through holes selectively linking the circuit baths on both sides of the substrate or (2) holes which provide for electrical connection with later assembled leads of components.
There are a variety of methods of manufacturing printed circuits such as substractive, semi-additive and fully additive. The subtractive method initially requires copper cladding on the substrate whereafter copper is removed to form the circuit paths. The semi-additive method requires adding a thin layer of copper to an unclad substrate and then the copper is etched away to form the circuit paths. The fully-additive method is an improvement over the previous two methods in that metal is only plated where it is desired. Thus, the costly process of removing copper from the substrate as well as the loss of copper in waste material is avoided.
In one method of manufacturing printed circuits using an electroless plating method, holes are formed through a copperless substrate which is then chemically cleaned. A thin ultraviolet sensitive film is applied to the major surfaces of the substrate which is subsequently exposed to an ultraviolet light through a positive art master of the desired circuit pattern. The substrate is then treated with a developer to remove those areas of the film which were not exposed to reveal an image of a filmless area in the desired circuit pattern. Subsequently, the substrate is chemically treated to roughen the surfaces of the substrate in the area of the desired circuit pattern. A palladium-bearing catalyst solution is applied to the surfaces of the substrate. The substrate is placed in an electroless copper solution for a period of one to five minutes to plate a thin porous copper layer over the palladium. This thin copper layer enhances the image and provides for longer storage of the substrate prior to ultimate copper plating. The substrate is then contacted by a stripper which removes the thin film, the palladium and copper from the exposed areas but leaves intact the copper deposited in the desired circuit pattern. The substrate is subsequently placed into an electroless plating bath of an aqueous solution containing cupric ions where copper is deposited in the circuit pattern to a sufficient thickness as dictated by product requirements.
This method of plating requires a controlled level of cupric ions in the bath to (1) insure a desired plating rate, (2) provide good metallic properties of the deposit and (3) minimize any tendency toward extraneous plating. The electroless plating process normally requires an alkaline solution which contains a cupric salt and has a high pH level. A complexing agent, such as ethylenediaminetetraacetate (EDTA), is added to the plating solution to prevent the precipitation of cupric hydroxide. In this use, complexer molarity will normally be greater than copper molarity. Normally in the absence of the complexing agent, cupric ions could be monitored by using a specific ion probe placed in the plating bath. However, complexed copper ions are not usually detectable with a specific ion probe.
U.S. Pat. No. 3,951,602, which issued to D. S. Thompson, discloses a system for continuously measuring copper ion concentration of an electroless copper plating bath and automatically replenishing the bath based on these measurements. The copper ion concentration of the bath is determined by a spectrophotometer which measures the amount of light absorbance by the bath at specific wavelengths. The copper ions are complexed and have a maximum absorbance at a known wavelength and pH level. The spectrophotometer measures the amount of absorbance of light at the known wavelength and compares it to the amount of absorbance of light at a reference wavelength to indicate the concentration of copper ions. An electrical signal representative of the copper ion concentration is generated by the spectrophotometer and transmitted to an electronic control and pump which replenishes the copper plating bath with a proper amount of copper sulfate solution.
The above method uses a colorimeteric technique. If a precipitate forms in the solution during the plating process, the amount of light absorbed is affected. This results in an erroneous measurement of the concentration of copper ions.
Consequently, there is a need for methods of and a system for controlling copper concentration in a solution which contains a complexer having a molarity greater than the copper molarity in a desired ratio.